A heterojunction bipolar transistor (HBT) is a type of bipolar junction transistor (BJT) which uses differing semiconductor materials for the emitter and base regions, or base and collector regions, thereby creating a heterojunction. The HBT can handle signals of very high frequencies, up to several hundred GHz. The HBT is commonly used in radio-frequency (RF) systems, and in applications requiring a high power efficiency, such as RF power amplifiers in cellular phones.
BiCMOS technologies with very good low-noise amplifier (LNA) and very good power amplifier (PA) SiGe HBTs are difficult to integrate on the same wafer due to different optimization requirements. For example, LNA devices need high current gain beta, low Rb (base resistance) and low Ccb (collector-base capacitance) for low Noise Figure (NF) and high gain. A self-aligned emitter/base integration scheme is preferred for the LNA as it results in both lower Rb and lower Ccb. In LNA technologies, an implanted extrinsic base structure is typically used which results in high Rb and high base current (low beta). On the other hand, the PA devices need low Cbe (base-emitter capacitance) and high BVceo (for a given beta). In such PA implementations, a wide emitter is required and self-alignment of emitter-base is not essential. Accordingly, in current integration schemes, only one device can be optimized for performance, with the performance of the other device mostly restricted.